The use of polymers in separation processes is well known in a variety of industries from water treatment to industrial gas separations. The polymers generally accomplish separation through a solution-diffusion mechanism when utilized as a non-porous material, and through molecular sieving or Knudson diffusion when utilized as porous materials. Additionally, polymers have been fabricated for a combination of porous and non-porous behaviors with a morphology characterized as asymmetric. An asymmetric polymer material is generally characterized by a dense thin film providing high selectivity, and supported by a porous polymeric network having high permeability. In polymer membranes utilized for gas separations, the asymmetric morphology is advantageous in that it allows a component selectively passed through the dense layer to proceed through the lower structure with minimized resistance.
In polymer membranes utilizing an asymmetric morphology, it is important that the preferred component diffuse through the dense layer at a sufficient flow rate, which makes it desirable to keep the thickness of the dense layer as small as possible. Additionally, typically the dense film cannot have a porous surface in order to preserve the inherent selectivity of the dense film in gas separation operations. These two requirements present significant difficulty in connection with the fabrication of the asymmetric morphologies, since dense films of a thickness of 1 micron or less can rarely be manufactured without flaws. Typically, the exceedingly small dimensions of gas molecules make the presence of even Angstrom size pores in the separation layer unacceptable. In order to combat this requirement for defect-free dense films, polymer structures are also utilized as supporting contactors between a gaseous mixture and a supported carrier or sorbent. Unlike the purely polymeric membranes, the sorbent-supporting polymer mainly provides structural support, since the composite material relies on the supported material to provide selectivity.
One particularly valuable geometry for the support and confinement of liquid carriers or solvents is the hollow polymeric microsphere. Spheres with hollow interiors play an important role in microencapsulation and have been extensively used in medical, biological, pharmaceutical and industrial applications. They offer the advantage of large internal payload space and high specific surface area, and the investigation of hollow polymer microspheres with different and controllable shell thickness has become increasingly important as a result of their superior mechanical properties and release behavior in medical and pharmaceutical applications. However, generally the hollow polymer microspheres fabricated for these applications are characterized by a densified outer polymer skin surrounding the hollow interior, rather than the asymmetric geometries desired for applications such as gas separations. In many applications, the hollow polymer microspheres are fabricated around a core fluid which is essentially inert with respect to the polymer dope forming the outer shell, so that the densified outer shell forms with limited interference. As a result, the hollow microspheres formed by these processes have limited use in applications such as gas separations.
It would be advantageous to provide a process whereby polymer microspheres comprised of an asymmetric layer surrounding a hollow interior could be fabricated, so that the support and confinement of liquid carriers or solvents could be accommodated within a spherical geometry incorporating the type of layer desired for polymeric membranes. It would be additionally advantageous if the asymmetric geometry of the hollow microsphere resulted from specified solvent-nonsolvent relationships within the core fluid, so that the advantages of a core fluid in fabricating the hollow interior could be utilized without interference in the formation of the asymmetric layer.
Ionic liquids have been encapsulated in polymeric microspheres through the preparation of a polymer-solvent-ionic liquid dope, and the immersion of contiguous drops of the dope into a coagulation bath in order to produce generally asymmetric surrounding layers. See Lakshmi et al., “Preparation and characterization of ionic liquid polymer microspheres [PEEKWC/DMF/CYPHOS IL 101] using the phase-inversion technique,” Separ. Purif. Technol. (2012), doi:10.1016/j.seppur.2012.01.045. This methodology constrains both polymer and ionic liquid concentrations within relatively tight margins in order to control the viscosity of the resulting dope and allow for formation of the contiguous dope spheroid under the method employed, and the absence of a core fluid generates limited control over the hollow geometry. Additionally, polymer microspheres comprised of an asymmetric layer have been fabricated using a volatizing core fluid under controlled temperature conditions to avoid microsphere collapse during fabrication. See Wickramanayake et al., “Fabrication of hollow, spherical polymeric micropillows using a dual layer spinneret,” Journal of Applied Polymer Science 121(5) (2011). It would be advantageous to provide a process whereby polymer microspheres comprised of an asymmetric layer surrounding a hollow interior could be fabricated in a logistically simpler wet-dry process through the control of core fluid and polymer dope compositions, as well as the resulting interactions between the polymer dope, the core fluid, and a surrounding environment during the formation.
Disclosed here is a method for the production of a fabricated hollow microsphere comprised of a densified outer layer surrounding a porous polymer network, where the porous polymer network surrounds a hollow interior. The resulting geometry of the fabricated hollow microsphere results from interactions between a polymer dope, a core fluid, and a gaseous environment when respective compositions, diameters, and dope thicknesses are combined with a controlled exposure time to a gaseous environment, and factors as specified in this disclosure are observed. The fabricated hollow microsphere may be fabricated in a wet-dry process utilizing a triple orifice, dual layer spinneret, and the fabricated hollow microspheres generated may be effectively utilized as liquid sorbent supports.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.